1. Field of the Invention
The present invention concerns an electrode for use in oxygen electrode reaction which is suitable as a cathode in a fuel cell such as an oxygen-hydrogen cell.
2. Description of the Prior Art
In an oxygen-hydrogen electric cell, porous hollow electrodes each made of graphite, platinum or nickel are inserted into an electrolyte solution and hydrogen is charged to one electrode (positive electrode) and oxygen charged to the other electrode (negative electrode), in which utilized the electric current due to the following reactions when both of the electrodes are connected by conductors:
Positive electrode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e
Negative electrode reaction: O.sub.2 +4H.sup.+ +4e.fwdarw.H.sub.2 O
In this case, when viewed as an electric cell, the electrode from which electrons flow out (positive electrode in the reaction above) constitutes a cathode, while the electrode into the electrons flow constitutes an anode (negative electrode in the reaction above).
The electromotive force in an oxygen-hydrogen cell is determined as an oxygen equilibrium potential. The oxygen equilibrium potential is defined as a value obtained by subtracting the standard hydrogen electrode potential at the anode from the potential under the equilibrium state at the cathode and the value theoretically determined from a thermodynamical point of view is 1.23 V vs NHE.
However, cathode materials capable of stably obtaining the oxygen equilibrium potential approximate to the theoretical value have not yet been obtained at present. For instance, no equilibrium potential can be obtained by a Pt/PtO.sub.2 electrode, and the static potential, at the most noble state, 1.15 V, io=10.sup.-9 A/cm.sup.2. In a Ru.sub.2 O.sub.3 or PbO.sub.2 carbon electrode, the static potential is: .ltoreq.0.9V, io=10.sup.-6 A/cm.sup.2 and it is stable only for several days.
It is considered that the reason why the oxygen equilibrium potential approximate to the theoretical value can not be obtained stably is due to the fact that dissociation and adsorption of oxygen is inhibited at the electrode surface when oxygen is intaken to the electrode at the cathode and that complicate reactions proceed at the surface of the electrode to change the potential.